1. Field of the Invention
This invention relates to a method of fabricating a light emitting device.
2. Description of the Related Art
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2003-218383
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2003-209283
[Patent Document 3] Japanese Patent Publication No. 2780744
[Patent Document 4] Japanese Laid-Open Patent Publication No. H8-115893
[Patent Document 5] Japanese Laid-Open Patent Publication No. H5-167101
[Patent Document 6] Japanese Laid-Open Patent Publication No. H8-102548
Light emitting device having a light emitting layer section thereof composed of an (AlxGa1-x)yIn1-yP alloy (where, 0≦x≦1, 0≦y≦1; simply referred to as AlGaInP alloy, or more simply as AlGaInP, hereinafter) can be realized as a high-luminance device over a wide wavelength range typically from green region to red region, by adopting a double heterostructure in which a thin AlGaInP active layer is sandwiched between an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer, both having a larger band gap than the active layer. Current is supplied to the light emitting layer section through a metal electrode formed on the surface of the device. The metal electrode acts as a light interceptor, so that it is formed, for example, so as to cover only the center portion of a main surface (that is defined as a first main surface hereinafter) as a light extraction side of the light emitting layer section, to thereby extract light from the peripheral region having no electrode formed thereon.
In this case, a smaller area of the metal electrode is advantageous in terms of improving the light extraction efficiency, because it can ensure a larger area for the light extraction region formed around the electrode. Conventional efforts have been made on increasing the amount of light extraction by effectively spreading current within the device through consideration on geometry of the electrode, but increase in the electrode area is inevitable anyhow in this case, having been fallen in a dilemma that a smaller light extraction area results in a limited amount of light extraction. Another problem resides in that the current is less likely to spread in the in-plane direction, because the dopant carrier concentration, and consequently the conductivity, of the cladding layer is suppressed to a slightly lower level in order to optimize emissive recombination of carriers in the active layer. This results in concentration of the current into the region covered by the electrode, and consequently lowers the substantial amount of light extraction from the light extraction region. There has been adopted a method of forming, between the cladding layer and the electrode, a low-resistivity current spreading layer having a dopant concentration larger than that of the cladding layer. The current spreading layer, formed as a light extraction layer increased in the thickness to a certain degree, is not only successful in improving the in-plane current spreading effect, but also in increasing extractable amount of light from the side faces of the layer to thereby raise the light extraction efficiency. It is necessary for the light extraction layer to be formed using a compound semiconductor having a band gap energy larger than a light quantum energy of the beam of emitted light, for the purpose of efficient transmission of the beam of emitted light and raising the light extraction efficiency. In particular, GaP is widely used for composing the light extraction layer of AlGaInP-base light emitting device, by virtue of its large band gap energy and small absorption of the beam of emitted light.
In thus-configured light emitting device using the peripheral region of the metal electrode on the first main surface of the light extraction layer as the light extraction area, not all components of the emitted light directed from inside of the device towards the light extraction area can be extracted, because some components incident on the light extraction area at an angle (angle of incidence herein means an angle of the direction of incidence of beam away from the normal line on the area) larger than the critical angle returns back inside the device by total reflection. The Patent Documents 1-3 disclose techniques of roughening (also referred to as frosting) the first main surface of the light extraction layer using an appropriate etching solution so as to form fine irregularities, aiming at reducing probability of incidence of the beam of emitted light at large angles and at consequently raising the light extraction efficiency. Also, as another technique, the Patent Documents 5 and 6 disclose a technique of epitaxially growing, on a current spreading layer, a light scattering layer composed of compound semiconductor having a different lattice constant from that of the current spreading layer, so as to form irregularities on the light scattering layer resulting from lattice mismatching of the both layers.
The Patent document 1, however, discloses that the surface roughening using the etching solution can roughen some surfaces but cannot roughen other surfaces depending on orientation of the exposed surface, and upper surface roughening is therefore not always successfully done, so that improvement in the light extraction efficiency is achievable only to a limited degree, and further improvement in the luminance is not easy. The Patent document 2 more specifically discloses that “the main surface of semiconductor substrate generally shows the {100} surface or a surface several degrees off-angled from the {100} surface, so that the surface of any of the individual semiconductor layers grown thereon has also the {100} surface or a surface several degrees off-angled from the {100} surface, wherein it is difficult to roughen the {100} surface and the surface several degrees off-angled from the {100} surface”. The light extraction layer disclosed in the Patent documents 2 and 3 is a GaAlAs layer, whereas the Patent document 1 discloses a GaP light extraction layer, having again the {100} surface exposed on the first main surface thereof.
Putting all aspects disclosed in the Patent documents 1-3 together, it is obvious that the GaP light extraction layer having the {100} surface on the first main surface thereof cannot be roughened simply by immersing it into the etching solution, so far as any publicly-known etching solution for GaP (hydrochloric acid, sulfuric acid, hydrogen peroxide or mixed solutions of these components, according to paragraph 0026 in the Patent document 1) is used as the etching solution, and that it is difficult to form the irregularities capable of improving the light extraction efficiency to a sufficient degree.
Also, GaP is III-V compound semiconductor having a zincblende structure, and has a crystal structure having closest packing surfaces of Ga and closest packing surfaces of P stacked alternately in the <111> direction. In this case, GaP single crystal having a principal axis orientation adjusted to exact <100> has the equal existence rate of Ga atoms and P atoms on the two main surfaces, however in a case of an off-angled {100} surface, the existence rate becomes unbalanced, so as that it is rich in Ga on the one main surface and rich in P on the other. The present inventors have studied and found that in the case of an off-angled {100} surface, surface roughening by chemical etching is especially difficult on the P-rich side.
The Patent Document 1 discloses a method of etching the {100} main surface of the GaP light extraction layer, after covering it with a finely-patterned resin mask. Although the document also formally suggests wet etching (chemical etching) as the etching method, all specific disclosures including the embodiments are made only on dry etching based on RIE (reactive ion etching). Dry etching costs high, and is disadvantageous in terms of its extremely low process efficiency, because only a small area of substrate can be processed at a time. Also, there is no specific method disclosed for effective surface roughening by chemical etching on a {100} surface off-angled so as to be rich in P.
On the other hand, the Patent Documents 2 and 3 give no specific information on the formation of the irregularities by etching the {100} main surface of the GaP light extraction layer, because the light extraction surface herein is composed of GaAlAs. A method adopted herein is such as forming a macroscopic secondary trench pattern having a triangle section by mechanical processing so as to expose the {111} surface allowing the etching to proceed thereon more easily, and subjecting the surface of the secondary pattern to chemical etching. The method is, however, disadvantageous in that the number of process steps increases corresponding to necessity of the mechanical processing for forming the grooves.
The light emitting device having the GaP light extraction layer can increase the amount of light extractable from the side faces of the GaP light extraction layer, if the layer is thickened. Therefore the light extraction efficiency of the device as a whole can further be increased by roughening also the side faces of thus-thickened GaP light extraction layer. The methods of surface roughening described in the Patent Documents 1-3, however, essentially need process steps of forming the mask and the trenches, which are permissible only on the main surface of the wafer, and this consequently raises a critical disadvantage in that the surface roughening is not adoptable to the side faces of the chip which appear only after the wafer is diced. In particular in the dry etching such as RIE as described in the Patent Document 1, it may be absolutely impossible to etch the side faces by allowing the etching beam, directed straightforward to the main surface of the layer, to come round onto the side faces, due to strong directionality of the etching beam. Further, the methods of surface roughening using epitaxial growth described in the Patent Documents 5 and 6 have a disadvantage that it is difficult to conduct surface roughening producing distinguished irregularities, comparing to methods by etching, and also that surface roughening to side faces of a chip is almost impossible. In these documents, there is no specific method disclosed for effective surface roughening by chemical etching on a {100} surface off-angled so as to be rich in P.
It is therefore a subject of this invention to provide a fabricating method of a light emitting device, allowing surface roughening to be easily applied to a GaP light extraction layer having a {100} surface, off-angled so as to be rich in P, as a main surface.